linux-kernel - Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE

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Date:   Thu, 11 Feb 2021 12:21:43 +0000
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        David Hildenbrand <david@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        linux-mm@...ck.org,
        Jérôme Glisse <jglisse@...hat.com>,
        James Morse <james.morse@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based
 memory

On Thu, Feb 11, 2021 at 05:40:35PM +0530, Anshuman Khandual wrote:
> On 2/11/21 5:23 PM, Will Deacon wrote:
> > On Fri, Feb 05, 2021 at 06:55:53PM +0000, Will Deacon wrote:
> >> On Wed, Feb 03, 2021 at 09:20:39AM +0530, Anshuman Khandual wrote:
> >>> On 2/2/21 6:26 PM, David Hildenbrand wrote:
> >>>> On 02.02.21 13:51, Will Deacon wrote:
> >>>>> On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote:
> >>>>>> As I expressed already, long term we should really get rid of the arm64
> >>>>>> variant and rather special-case the generic one. Then we won't go out of
> >>>>>> sync - just as it happened with ZONE_DEVICE handling here.
> >>>>>
> >>>>> Why does this have to be long term? This ZONE_DEVICE stuff could be the
> >>>>> carrot on the stick :)
> >>>>
> >>>> Yes, I suggested to do it now, but Anshuman convinced me that doing a
> >>>> simple fix upfront might be cleaner --- for example when it comes to
> >>>> backporting :)
> >>>
> >>> Right. The current pfn_valid() breaks for ZONE_DEVICE memory and this fixes
> >>> the problem in the present context which can be easily backported if required.
> >>>
> >>> Changing or rather overhauling the generic code with new configs as proposed
> >>> earlier (which I am planning to work on subsequently) would definitely be an
> >>> improvement for the current pfn_valid() situation in terms of maintainability
> >>> but then it should not stop us from fixing the problem now.
> >>
> >> Alright, I've mulled this over a bit. I don't agree that this patch helps
> >> with maintainability (quite the opposite, in fact), but perfection is the
> >> enemy of the good so I'll queue the series for 5.12. However, I'll revert
> >> the changes at the first sign of a problem, so please do work towards a
> >> generic solution which can replace this in the medium term.
> > 
> > ... and dropped. These patches appear to be responsible for a boot
> > regression reported by CKI:
> 
> Ahh, boot regression ? These patches only change the behaviour
> for non boot memory only.

Sure, but this thing is horribly fragile, which is why I was nervous about
touching it in the first place ;)

> > https://lore.kernel.org/r/cki.8D1CB60FEC.K6NJMEFQPV@redhat.com
> 
> Will look into the logs and see if there is something pointing to
> the problem.

We don't have a log yet, but I've asked whether earlycon works on the
problematic machine (the failure seems to be specific to a certain TX2).

Either way, this is too late for 5.12 now.

Will